The use of the piezoresistive effect in silicon as the transduction basis for a semiconductor stress or pressure sensor is well known and such stress sensors are widely used. Such semiconductor stress sensors are disclosed, by Marshall in U.S. Pat. Nos. 4,035,823 and 4,125,820 entitled, "Stress Sensor Apparatus" which are both assigned to the present assignee. Typically, a p-type conductivity region is formed in an n-type conductivity silicon layer by diffusion or ion implantation techniques so that this p-type region serves as a pn junction isolated resistor which forms the semiconductor stress sensor, i.e. a piezoresistor. Usually, this resistor is provided in and on a semiconductor body comprising a substrate having structural portions including both a diaphragm portion and a constraint portion for constraining the diaphragm at peripheral portions thereof. The pn junction isolated resistor is located at least in part in the diaphragm portion. The diaphragm in operation is exposed to a source of stress. By electrically contacting the p-type region, or piezoresistor, at two points and measuring the resistance changes between these two contact points before and during applications of stress, the magnitude of applied stress can be determined because of the known piezoresistive response of the piezoresistor in the diaphragm portion of the substrate.
The semiconductor stress sensor, or piezoresistor, performance characteristics are strongly dependent upon (a) the dopant distribution therein, including the maximum dopant concentration in the piezoresistor; and (b) the mechanical structures supporting the piezoresistor, i.e. its placement in the diaphragm as part of the diaphragm.
The total resistance value of the piezoresistor at any given temperature is, of course, related to the total amount of dopant atoms provided in the region in which it is formed. On the other hand, the temperature coefficient of resistance (TCR) of the piezoresistor is related primarily to the maximum dopant concentration occurring in this same region.
The change in the piezoresistor resistance value as a function of temperature at a constant applied stress condition can, of course, be described in fractional terms by the piezoresistor TCR, .differential.R/.differential.T/R(T0), or in absolute units simply by .differential.R/.differential.T. The change in resistance value as a function of stress at a constant ambient temperature is the stress sensor transduction performance, .differential.R/.differential.(stress), herein referred to as Pi.
The stress sensor transduction performance, Pi, over temperature has been found to be primarily related to the maximum dopant concentration value in the piezoresistor region. The temperature dependent transduction performance is the resistance change versus applied stress, .differential.R/.differential.(stress) as a function of temperature .differential..sup.2 R/.differential.T.differential.(stress) or in other words, the piezoresistive stress sensitivity temperature coefficient, herein referred to and identified as the Pi temperature coefficient. Herein, Pi characteristics refers to both Pi and its temperature coefficient.
The piezoresistor performance characteristic, as said earlier, is strongly dependent on the mechanical mechanism supporting the piezoresistor, i.e. the diaphragm and constraint structure. A thermally induced stress applied to the diaphragm portion in the stress sensor substrate results from temperature changes in the piezoresistor mechanical supporting mechanism. This thermally induced stress results in a shift (SHIFT) in the piezoresistor performance characteristic with changing temperature at constant applied stress.
Advances in semiconductor fabrication technology, specifically in ion-implantation techniques, have made it possible to accurately control the dopant distribution in a resistor, both the total dopant in a pn junction isolated resistor formed in silicon, and also, the maximum dopant concentration in such a resistor. These fabrication advances allow production of piezoresistive sensors having TCR and Pi characteristics which are repeatable and reproducible from device to device so that temperature compensation schemes may be implemented. Further, making the diaphragm of silicon in the manner shown by Marshall in U.S. Pat. Nos. 4,035,823 and 4,125,823 has substantially reduced and made repeatable the thermally induced stress in the supporting mechanical structure so as to make compensation schemes practical. The above referenced patents disclose semiconductor stress sensors which have substantially device to device reproducibility and repeatable temperature characteristics through controlling the depth, or location, of, and the value of maximum dopant concentration of the resistor dopant, and through controlling the mounting of the sensor when affixed to mounts of a material type differing from the sensor material. However, many applications require improved temperature dependent transduction performance characteristics of the stress transducer system over extended temperature ranges above that which can be provided by known semiconductor stress sensor processing techniques.
As stated earlier, production of piezoresistive sensors having TCR and Pi temperature and stress characteristics, respectively, which are reproducible and repeatable from device to device make possible the use of general temperature compensation schemes to provide improved performance characteristics for the stress transducer system over the desired extended temperature ranges. Many of the compensation schemes presently known have disadvantages in that they require special components or are not easily fabricated in monolithic integrated circuit chips, or both.